1. Field of the Invention
The present invention relates to an aqueous ink composition, a manufacturing method therefor and an image forming method using said aqueous ink composition.
2. Description of the Related Art
Along with the recent increased awareness for environmental conservation, the application of aqueous media in place of organic solvents has been recommended and even legally imposed in various fields.
For example, in the paint field, there has been a strong demand for a switch from organic solvent-type paints to water-based paints and non-solvent-type paints (electrostatic powder paints, etc.). Moreover, in the printing field as well, there are demands for legal restrictions to impose the application of aqueous print inks in order to improve areas such as printing work environments. Hence, there has been a rush to develop aqueous print inks. However, aqueous inks are problematic in that pigment dispersion is still insufficient, a problem that has not been completely solved, and at present, images formed by using an aqueous print ink have problems in fastness, and, in particular, poor water-resistance. Accordingly, the current techniques do not provide adequate image properties for practical use. At present, in the case when a non-solvent-type ink needs to be obtained, an aqueous print ink with inferior image characteristics has to be used. Therefore, there is a need to further develop aqueous print inks having sufficient image characteristics.
An example of another recent application of aqueous inks is in image recording processes using ink-jet recording ink. Despite its simple structure and low power consumption, the ink-jet process makes it possible to record high-quality images, and in particular, recent advances have provided images that are equivalent to photographs in image quality.
With ink-jet recording, an aqueous ink is discharged through a nozzle in the form of fine droplets so as to adhere to a recording medium; thus, a recording process is carried out. Conventionally, dyes are used as coloring materials for such ink solutions, and an aqueous solution, which excels from the standpoint of surface tension, is generally selected as the ink medium in order to form an even solution system. The dye used as the coloring material is highly water-soluble and less susceptible to nozzle clogging than other materials, and the ink solution using such a dye has stable ink-discharging properties since its characteristics are similar to those of Newtonian fluid. Another advantage such dyes possesses is that a wide range of colors are available.
However, in general, water-soluble dye possesses inferior water resistance and light resistance when compared with pigment, hence an image recorded with ink containing a dye also has inferior water and light resistance. Such images also easily run. Moreover, most of these dyes are highly water-soluble, and tend to have strong electron bonds in their chemical structures, with the result that they tend to be ahrmful to humans. Moreover, ink-jet recording processes using dye-containing inks have failed to achieve sufficient color reproducibility on regular paper (although sufficient color reproduction range is obtainable if special paper is used).
Since ink-jet recording processes have come to be widely used, there has been an ever-increasing demand for improved recording characteristics, to the point that the same performance level as that of printed images has become required of ink-jet recorded images. Therefore, there is an urgent need to solve the above-mentioned problems regarding water and light resistance, color running, color reproducibility and safety, which are inherent problems with dye-containing ink.
Here, with respect to ink-jet recording ink using a pigment as the coloring material, various aspects have been developed and researched, specifically with polymer dispersants and aqueous solvents. However, at present, dispersants and solvents having sufficient characteristics have yet to be obtained.
For example, patent document 1 (identified below) discloses an ink-jet recording ink having improved clogging resistance, which serves as an ink containing a pigment, and contains an aqueous carrier medium, a nitrogen heterocyclic diol assistant solvent and a pigment dispersant (an aqueous dispersion of pigment particles that are stabilized by a polymer dispersant). Patent document 1 also discloses an AB or BAB block copolymer as a polymer dispersant (the A segment is an acrylic hydrophobic polymer, and the B segment is an acrylic hydrophilic polymer).
Moreover, patent document 2 (identified below) discloses an arrangement in which, in a recording solution containing a pigment, saccharides are used as moisturizers in place of ethylene glycol and the like so that the same moisture-retaining performance is achieved at a lower viscosity. Patent document 2 also discloses use of a copolymer of a monomer that is at least one member selected from the group consisting of styrene and vinyl naphthalene and their derivatives and further discloses using an unsaturated carboxylic acid such as acrylic acid is used as a polymer compound serving as a dispersant of the pigment.
Furthermore, patent document 3 (identified below) has disclosed that, with respect to a pigment dispersant of an aqueous ink-jet ink containing a pigment, an aqueous ink-jet ink having a stabilized pigment dispersing property is obtainable by using a block polymer composed of a hydrophobic polymer block having at least one basic amine functional group and a nonionic polymer block or a polymer block containing an acidic functional group.
Patent document 4 (identified below) discloses an ink-jet recording ink in which a pigment is dispersed by a polymer dispersant such as a styrene-methacrylic acid-acrylate copolymer.
All of the ink-jet recording inks disclosed in the above-mentioned patent documents contain a polymer compound (binder resin) as a dispersant for pigments, and the polymer compound contains an acidic group for assisting the dissolving property to water and a hydrophobic group having a high affinity for a pigment. Here, the above-mentioned polymer compound, which not only serves as a dispersant for a pigment but also has a function for making the pigment adhere to the recording medium, also makes it possible to prevent light diffusion on the particle surface caused by the pigment. Prevention of light diffusion is accomplished by coating the pigment with a polymer compound coating film, which consequently functions to improve the reflection density of the image.
Therefore, in order to improve the color density of a recorded image, it is preferable not only to increase the pigment concentration in the ink, but also to increase the polymer coating property on the pigment surface by increasing the content of a polymer compound in the ink.
However, in the case when a great amount of the polymer compound disclosed in any one of the above-mentioned patent documents 1 to 4 is added to the ink, the ink viscosity increases, making it impossible to apply as an ink-jet recording ink. Moreover, another problem is that an image recorded by using an ink disclosed in any one of the above-mentioned patent documents 1 to 4 has inferior resistance to water.
The above-mentioned patent documents are: patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 5-214284 (paragraphs 0016, 0037-0053); patent document 2: JP-A No. 7-11182 (claims, paragraphs 0010, 0016-0025); patent document 3: JP-A No. 6-136311 (paragraphs 0016-0025); patent document 4: JP-A No. 2002-38061 (paragraphs 0051, 0054, 0102).